1. Field of the Invention
The present invention relates to a device for detecting a rotation angle of a rotating object.
2. Description of the Related Art
A rotation angle detecting device for detecting a rotation angle of a rotating object is well-known. For example, such a rotation angle detecting device is installed into an ignition device of an internal combustion engine to detect the rotation angle of an engine crank shaft. However, the internal combustion engine may fail if the rotation angle detecting device fails.
In order to prevent the above problem, JP-A-2002-104211 proposes to provide a rotation angle detecting device with an abnormality detecting function to be used for a motor-driven power steering system. This rotation angle detecting device includes a motor-rotation-angle sensor of a steering-shaft-driving motor and a steering-angle sensor of a steering shaft, so that the abnormality can be detected by comparing the motor-rotation-angle with the steering-angle. That is, in order to prevent the above-stated problem, the rotation angle detecting device disclosed in JP-A-2002-104211 necessitates a motor-rotation-angle sensor in addition to the steering-angle sensor.